fix: add UnityPostMessage alias for macOS WebView compatibility

The game-bridge JavaScript checks for UnityPostMessage before window.Unity.call.
This commit adds UnityPostMessage as an alias to ensure proper callback communication
on macOS, allowing the 'ready' signal to be received correctly.

Author: rodrigo-fournier-immutable

Plugins/Mac/Sources/ImmutableWebView.mm

@@ -118,6 +118,7 @@ - (id)initWithUa:(const char *)ua
             window.webkit.messageHandlers.saveDataURL.postMessage(fileName + '\t' + dataURL); \
         } \
         }; \
+        window.UnityPostMessage = window.Unity.call; \
         function captureLog(msg) { window.webkit.messageHandlers.logHandler.postMessage(msg); } window.console.log = captureLog; \
         ";